The !HealPix concept (and the original libraries) are described at https://healpix.jpl.nasa.gov/index.shtml.

A !HealPix implementation for ROOT is available at https://github.com/akira-okumura/RootHealPix. You can compile that into your Mars libarary.

After checking out Mars from the repository (see InstallingMars), simple extract the attached zip-file into your Mars source directory or get !RootHealPix from git:

{{{
[0] cd Mars
[1] git clone https://github.com/akira-okumura/RootHealPix.git
}}}

If !RootHealPix is located in your Mars source directory, cmake should find it automatically and compile the healpix classes into the Mars library, otherwise call cmake with the corresponding path:

{{{
[2] cmake -DHEALPIX_PREFIX=<path-to-RootHealPix>
}}}

To test the installation, start root in your Mars build directory and try something like:

{{{#!c++
THealPixD hp("name", "", 2); 
for (int i=0; i<10000; i++) 
   hp.Fill(gRandom->Uniform()*90, gRandom->Uniform()*180); 
hp.Draw("COLZ");
}}}

The documentation is not very complete, but you can find it here:
https://www.fact-project.org/RootHealPix/.

The example above should produce something like this:
[[Image(healpix-example.png)]]